Integrated circuit apparatus, systems, and methods

ABSTRACT

High density circuit modules are formed by stacking integrated circuit (IC) chips one above another. Unused input/output (I/O) locations on some of the chips can be used to connect other I/O locations, resulting in decreased impedance between the chips. Additional apparatus, systems, and methods are disclosed.

RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 13/208,986, filed 12 Aug. 2011, which is a continuation applicationof U.S. application Ser. No. 12/197,869, filed 25 Aug. 2008, now U.S.Pat. No. 8,001,513, which applications are incorporated, by referenceherein in their entirety.

BACKGROUND

The semiconductor device industry has a market driven need to reduce thetime required for signals to travel between integrated circuits (ICs),such as the high number of relatively low cost memory chips used invirtually every electronic device. One method known to reduce signaltravel time is to reduce the physical distance between closely relatedIC chips by attaching them together in a vertical stack. This reducesthe distance the signals travel as well as reducing electricalresistance, inductance and capacitance, resulting in faster systemsusing IC technology.

There is an industry wide issue in stacking ICs such as memory chips andlogic chips, and interconnecting the ICs to reduce the capacitance forfast inter-chip communication. Current methods of connecting similarInput/Output (I/O) pads on stacked chips, such as using wire bonds,share all of the I/O pads for all of the chips in the vertical stack.This may increase the capacitance between the interconnected I/O pads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art stack of integrated circuit (IC) chips;

FIG. 2 illustrates a stack of IC chips connected in accordance with anillustrative embodiment;

FIG. 3 is a block diagram of an electronic device in accordance with anembodiment of the invention;

FIG. 4 is a diagram of an electronic system having devices in accordancewith an embodiment of the invention; and

FIG. 5 is a flowchart of a method to reduce impedance between connectedintegrated circuit devices in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific aspects and embodiments inwhich the present invention may be practiced. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the present invention. Other embodiments may be utilized andstructural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The variousembodiments are not necessarily mutually exclusive, as some embodimentscan be combined with one or more other embodiments to form newembodiments.

The term “substrate” as used in the following description may includeany structure having an exposed surface with which to form an ICstructure. The term “substrate” is understood to include semiconductorwafers and is also used to refer to semiconductor structures duringprocessing and may include other layers that have been fabricatedthereupon. A “wafer” and a “substrate” each include doped and undopedsemiconductors, epitaxial semiconductor layers supported by a basesemiconductor or insulator, as well as other semiconductor structureswell known to one skilled in the art. The term “chip” is used to referto IC devices, including logic devices, microprocessors, and varioustypes of memory devices.

The term “conductor” is understood to generally include n-type andp-type semiconductors and doped regions in semiconductors. Conductorsmay conduct electrical energy including ground potential, variousvoltage reference levels, and information signals, and may be referredto as wires, lines, cables, buses, planes or traces, depending upon thephysical configuration and use of the conductor. The term “insulator” or“dielectric” is defined to include any material that is lesselectrically conductive than the materials referred to as conductors oras semiconductors.

The term “crystalline” is understood to not be limited to large singlecrystals having a specified crystallographic orientation, but mayinclude polycrystalline materials having a large number of moderatelysized crystals having various crystallographic orientations. The term“amorphous” is not limited to a solid material having a completelydisordered or glassy structure, but may include materials having somecrystalline order over short distances—on the order of ten atomicseparations or less.

The term “horizontal” is defined as a plane parallel to the conventionalplane or surface of a wafer or substrate, regardless of the orientationof the wafer or substrate. The term “vertical” refers to a directionperpendicular to the horizontal as defined above. Prepositions, such as“on”, “side” “higher”, “lower”, “over” and “under” are defined withrespect to the conventional plane or surface being on the top surface ofthe wafer or substrate, regardless of the orientation of the wafer orsubstrate. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims, along with the full scope ofequivalents to which such claims are entitled.

FIG. 1 illustrates a prior art stack 100 of integrated circuit (IC)chips. Memory chips such as dynamic random access memory (DRAM), staticrandom access memory (SRAM), content addressable memory (CAM), and flashmemory, may be stacked together with similar input output (I/O) padshaving a similar function located directly above one another. It mayalso be convenient to directly connect similar function I/O pads to oneanother in a series connection of inter-connections, such as wire bonds.The stack 100 of ICs may include a number of memory chips 104, 106 and108, which may be located over a logic chip 102. The illustrated exampleincludes three memory chips, but there is no specific limit to thenumber of memory chips, nor is there any limitation on the types ofchips, which may include various types of memory chips, logic chips,microprocessor chips, or other types of chips.

The logic chip 102 has I/O pads 110, 112, 114, and 116 shown located onone side of the chip. Any of the four sides of a chip may have I/O padssuch as those shown in FIG. 1 and the I/O pads may be formed of two ormore connected metallic pads to allow for additional externalconnections, such as the shown wire bonds 140, 142, 144 146, 148 and150. Other connection methods may be used including gold bumps, solderbumps, beam leads, and other well known methods. In the example IC stack100, only I/O pads 110 and 114 are bonded and in use for interconnectionwith either the external world or the other chips 104, 106 and 108 inthe stack 100. I/O pads 112 and 116 are not used in the particulardesign shown in the figure, and thus the space for the unused I/O padsand the space for the connected power device is wasted in thisillustrative design.

In this arrangement the chip 102 provides power from I/O pad 110 todrive a signal through wirebond 140 to chip 104, through wirebond 142 toI/O pad 126 on chip 106, and through wirebond 146 to I/O pad 134 on chip108. The number and length of the wire bonds that the I/O pad 110 drivesto transmit signals to the three shown chips 104, 106 and 108 mayrequire a larger drive device connected to I/O pad 110 than is usual,and thus may require a specially designed chip rather than anoff-the-shelf device with a normal drive device circuit.

The I/O pad 114 on chip 102 is shown with another prior art stacked ICinterconnection method where the I/O pad 114 drives a wire bond 144 todrive a signal to I/O pad 122 on chip 104. The wire bond 148 connectingI/O pad 122 on chip 104 with I/O pad 130 on chip 106 has the same signalprovided in series from chip 102, and that signal is also driven by theI/O pad 114 on chip 102. I/O pad 130 on chip 106 is also connected bywire bond 150 to I/O pad 138 on chip 108, and the signal is again drivenby the same drive device circuit in I/O pad 114 on chip 102, resultingin a large impedance presented to the signal on its way to reach chip108. It should also be noted that I/O pads 120 and 124 on chip 104, I/Opads 128 and 132 on chip 106, and I/O pad 136 on chip 108 do not haveany wire bond connections in shown, as is the case for the unused I/Opads 112 and 116 on chip 102.

Each of the I/O pads 110, 112, 114 and 116 may have an attached outputtransistor to provide sufficient electrical power to overcome theimpedance of the interconnection wiring. Impedance is a measure of thedelay that charging up an electrical wire causes in the transmission ofa signal, and is a combination of the resistance, capacitance andinductance of the interconnect, such as the wire bonds 140, 142 and 144shown. In order to obtain sufficient signal transmission speed theoutput transistor may require a large size and use a large portion ofthe chip area.

FIG. 2 illustrates a stack 200 of IC chips connected according to anillustrative embodiment. Multiple interconnected IC devices, such asmemory devices and logic devices, are shown in a stacked arrangement.The stack 200 includes IC chips 202, 204, 206 and 208. Chip 202 maycomprise a logic chip or a microprocessor closely connected to memorychips 204, 206 and 208, but the present subject matter is not solimited, and may include any number of chips and any combination of chiptypes. As previously discussed, embodiments may use a base logic chip,such as chip 202, as the driver for signals transferred to memory chips204, 206 and 208. Thus the drive device connected to an I/O pad on chip202, such as pads 210, 212, 214 and 216, should be capable of driving astrong enough signal to rapidly overcome the impedance of inter-chipelectrical connections, such as wire bond 240 to I/O pad 218 on chip204, as well as the additional wire bonds and connections shown inFIG. 1. The present subject matter includes limiting the number of I/Opads on other chips in direct communications with I/O pad 210 of chip202. FIG. 2 shows that I/O pad 210 on chip 202 is electrically connectedto I/O pad 218 on chip 204, thus limiting the total electrical impedanceby limiting the number of other I/O pads in direct contact.

High density modules were previously formed by vertically stackingintegrated circuit (IC) die (chips) one above another. Each IC in thestack may have a number of input output (I/O) locations (or pads), someof which may not be used in a particular IC design. Each I/O pad on eachIC die might be connected (for example by wirebonding) to the I/O padimmediately below and the I/O pad immediately above, resulting inrelatively long signal propagation times. This situation may beaggravated by long wire lengths from the drive device on the sendingchip, which can be located in a substrate or on a controlling chip atthe bottom of the vertical stack of ICs. The long wire length and thecapacitive loading of the circuitry attached to each I/O pad may thusresult in increased impedance and increased signal propagation delay.The method of connecting chips in a stack shown in FIG. 2 provides lowerimpedance (i.e., at least one of low capacitance and low inductance)inter-chip connections by routing inter-chip signals intended to travelto more than a single IC through drive circuits at the unused I/Olocations. The drive circuit may also be beneficially located in a chiplocated in the middle of the stack, for example on chip 206 rather thanon chip 202, to reduce the total length of the worst case wire length,and therefore the time required for the signal to travel between thechips.

As previously discussed with regard to current methods ofinterconnecting stacked IC chips, not every single I/O pad on a chip islikely to be used in every design, and thus a number of I/O pads, suchas 112 and 116 on chip 102 of FIGS. 1, and 120 and 124 on chip 104 ofFIG. 1 are not in use in the particular electrical design shown. Thepresent arrangement can make use of these unused I/O pads as shown bythe wire bond 242 going from I/O pad 220 on chip 204 to I/O pad 228 onchip 206. The drive device associated with I/O pad 210 of chip 202provides the power for the signal from I/O pad 210 on chip 202 viawirebond 240 to I/O pad 218 on chip 204. Control circuitry associatedwith I/O pad 218 may then direct the signal to previously unused I/O pad220 (which is shown as being directly next to the I/O pad 218 but thepresent subject matter is not so limited and the previously unused I/Opad may be located anywhere on the chip 204), which may then use itsassociated drive device to transmit the signal from I/O pad 220 viawirebond 242 to the previously unused I/O pad 228 on chip 206. Controlcircuitry associated with I/O pad 228 may then direct the signal to I/Opad 226, which may use it's associated drive device to transmit thesignal via wire bond 246 to I/O pad 234 on chip 208. In this fashion thesame signal may be passed to each and every chip in a stack 200 withoutusing a single I/O pad device driver to provide all of the power neededto transmit the signal to all the other chips. This reduces the overallimpedance that a signal may experience by utilizing existing andotherwise unused device drivers and I/O pads to increase the datatransmission rate. The additional control circuitry used to direct theabove noted signal, if not already available in the standard chipdesign, may increase the size of a chip from about 0.5% to about 1.0%,depending upon the level of integration of the chips 202, 204, 206 and208.

Some embodiments make use of the unused I/O pads, and their associateddevice drivers and control circuitry to buffer communications to theoutside world, including other chips located in a stacked arrangementand closely coupled to a primary chip, and the printed circuit board(PCB) upon which the primary chip may be connected. The signal delay canthus be reduced by using previously unused I/O pads, the associatedcontrol circuitry and device drivers to reduce the number of other I/Opads and external circuits to which each I/O pad is directly connected.It should be noted that while the described arrangement of FIG. 2 showsno more than one electrical connection (such as wire bonds 240, 242,244, 246, 248 and 250) from each I/O pad (such as 210, 214, 218, 220,222, 224, 226, 228, 230 232, 234 and 238) as compared to the multiplewire bonds shown in FIG. 1, the present subject matter is not solimited. The FIG. 2 arrangement may be considered as a best casesituation when there are sufficient numbers of previously unused I/Opads to have a one-to-one correspondence between unused I/O pad driversand I/O pads that can benefit from being coupled to them. A goal is touse the unused I/O pads and drivers to obtain the shown situation of asingle connection from each I/O pad.

In the case where an I/O pad and it's associated device driver onlydrives a single communications signal such as a wire bond, the impedanceenvironment would appear to be approximately the same as in a monolithicdevice, and the resulting signal delays will be minimized as compared toa stack of ICs being driven by a single device driver and I/O pad. Thepresent subject matter may include an approximately 1% increase in chiparea related to adding control circuitry to control the routing of thesignals to the previously unused I/O pads.

FIG. 3 is a block diagram of an electronic device 306 in accordance withan embodiment of the invention. Electronic system 300 includes acontroller 302, a bus 304, and an electronic device 306, where bus 304provides electrical conductivity between controller 302 and electronicdevice 306. In various embodiments, controller 302 and/or electronicdevice 306 include an embodiment for a portion of the device 306 havinglogic and memory chips stacked and interconnected as previouslydiscussed herein. Thus, the device 306 may include one or more chipstacks that are similar to or identical to the stack 200 shown in FIG.2. Electronic system 300 may include, but is not limited to, informationhandling devices, wireless systems, telecommunication systems, fiberoptic systems, electro-optic systems, and computers.

FIG. 4 is a diagram of an electronic system 400 having devices inaccordance with an embodiment of the invention. The system 400 includesa controller 402 and a memory 406. Controller 402 and/or memory 406 mayeach include a potion of the circuit having IC devices and memory chipsstacked and connected in accordance with the disclosed embodiments.Thus, the controller 402 and memory 406 may include one or more chipstacks that are similar to or identical to the stack 200 shown in FIG.2.

System 400 also includes an electronic apparatus 408, and a bus 404,where bus 404 may provide electrical conductivity and data transmissionbetween controller 402 and electronic apparatus 408, and betweencontroller 402 and memory 406. Bus 404 may include an address bus, adata bus, and a control bus, each independently configured. Bus 404 alsouses common conductive lines for providing address, data, and/orcontrol, the use of which may be regulated by controller 402. In anembodiment, electronic apparatus 408 includes additional memory devicesconfigured similarly to or identically to memory 406. An embodimentincludes an additional peripheral device or devices 410 coupled to bus404. In an embodiment controller 402 is a processor. Any of bus 404,electronic apparatus 408, and peripheral device or devices 410 mayinclude ICs being stacked in accordance with the disclosed embodiments.Thus, the bus 404, the electronic apparatus 408, and the peripheraldevice 410 may include one or more chip stacks that are similar to oridentical to the stack 200 shown in FIG. 2.

System 400 may include, but is not limited to, information handlingdevices, telecommunication systems, and computers. Peripheral devices410 may include displays, additional memory, or other control devicesoperating with controller 402 and/or memory 406.

FIG. 5 is a flowchart of a method to reduce impedance between connectedintegrated circuit devices in accordance with an embodiment of theinvention. The reduction of impedance presented to signals propagatingwithin a chip stack may be obtained in various combinations of thedescribed embodiments, including a method of electrically connecting aplurality of integrated circuit devices 502, by first determining whichelectrical connections (preferably the input/output or I/O pads) on eachintegrated circuit device are not used in the circuit design 504, andwhich electrical connections or I/O pads are used. Then determiningwhich electrical paths connecting the ICs or chips in the stack have thelargest electrical impedance value 506 and interconnecting the chips byrouting portions of the highest impedance path 508 through some of theunused electrical connections 510 to spread the electrical load.Repeating this process 512 if there are still unused I/Os pads 514 tothe next I/O path 516 until there are no longer any unused I/O pads 518.In the best case there may be enough unused I/O pads to be able toarrange routing so that only a single electrical pad driver connectsfrom each electrical interconnection in a particular electrical signalpath. In some embodiments, the routing for the largest electricalimpedance value path may be arranged to have a single electricconnection between each I/O pad along the signal path. In the case wherethe chips already have sufficient control circuitry associated with theelectrical connection pads to route the electrical path to one of theunused electrical connections, there will be no area penalty torerouting signals through the unused I/O pads. In some cases, however,the extra control circuitry may add about 0.5 to 1.0% to the circuitryarea used on the chip.

In another method of interconnecting ICs to reduce the signal impedancein ICs attached in a vertical stack, the I/O pads of one IC connected(at least electrically) to another IC by connecting each I/O pad to onlyone other IC in the vertical stack. The I/O pads may include aconnection to an unused connection of another one of the ICs in thevertical stack, and the electrical connection may include wire bondswhich may be on at least one edge of the ICs. A stack of ICs may havethe IC with the largest area the lowest in the vertical stack, with asecond largest area one of the plurality of ICs the next higher IC inthe vertical stack, and a third largest area IC being the third IC inthe vertical stack, and located above the second largest area IC. In thecase where the ICs are not all in descending size order it may bebeneficial to form vertical bumps attached either between two facing topsurfaces of the ICs in the vertical stack, or connecting a top surfaceof a lower IC to input/output pads on a bottom surface of an upper IC.The top to bottom attachment may benefit from (and include) the use ofthrough silicon vias that go from the top surface to the bottom surfaceof the IC.

Another method of electrically connecting a plurality of IC devicesincludes determining a plurality of unused electrical connections oneach IC. Then determining a first set of electrical paths between theICs that have higher electrical impedance value than other electricalpaths, and could benefit from reduced impedance. Adding additionalinterconnections for the first set of paths by adding some of the unusedelectrical connections into the first set of paths to reduce theimpedance.

A circuit having the described arrangement of low impedanceinterconnections includes ICs attached in a vertical stack where eachindividual IC includes both electrical connections used in a selectedcircuit design and electrical connections not used in the circuitdesign. The electrical path connecting the ICs includes at least one ofthe otherwise unused electrical connections on at least one IC to spreadthe electrical load and utilize otherwise unused I/O pads and theirassociated drive circuitry. The interconnection may be more easilyconstructed if each one of the ICs in the vertical stack has a differentphysical dimension and can be stacked in decreasing size order with thelargest physical dimension located at the bottom and the smallestphysical dimension located at the top of the vertical stack.

Each of the ICs in the vertical stack includes a plurality of signaloutput pads on the edge for electrical connections, and the signaloutput pads may be on all four sides of the IC. The interconnection arearranged to reduce impedance by setting each of the signal output padsto be connected to only one of the other ICs in the vertical stack.Control circuitry on the signal output pads of a first IC may be used toroute an incoming signal to an unused signal output pad for transmissionto a second IC, and so utilize the unused portions of the first IC.

The signal output pads may be connected to selected other signal outputpads on other ICs by wire bonds, flexible tape, solder bumps, or goldbumps, depending upon the relative sizes of the various ICs and thetechnology, and it may be beneficial to limit each electrical connectionto a single connection to another IC.

A system having the described arrangement of low impedanceinterconnections may include a controller connected to an electronicdevice having a plurality of ICs coupled to the controller. Each of theICs may have electrical connections to the other integrated circuits.Each IC may have I/O pads and drive circuits used in a selected circuitdesign, and I/O pads and drive circuits not used in the selected circuitdesign. Additional inter IC connections using the I/O pads that areunused and added to the electrical path may reduce the overallimpedance. In this fashion the electrical path connects each one of theplurality of integrated circuits to each other one of the plurality ofintegrated circuits with a single electrical path similar to thesituation found in monolithic device.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of embodiments of thepresent invention. It is to be understood that the above description isintended to be illustrative, and not restrictive, and that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Combinations of the above embodimentsand other embodiments will be apparent to those of skill in the art uponstudying the above description. The present invention includes any otherapplications in which embodiments of the above arrangements are used.

The Abstract of the Disclosure is provided to comply with 37C.F.R.§1.72(b), requiring an abstract that will allow the reader toquickly ascertain the nature of the technical disclosure. It issubmitted with the understanding that it will not be used to interpretor limit the scope or meaning of the claims. In addition, in theforegoing Detailed Description, it can be seen that various features aregrouped together in a few embodiments for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted torequire more features than are expressly recited in each claim. Rather,inventive subject matter may be found in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate embodiment. The scope of the embodiments of thepresent invention should be determined with reference to the appendedclaims, along with the full scope of equivalents to which such claimsare entitled.

1. An electronic apparatus comprising: integrated circuit chips arranged in a stack; a first integrated circuit chip of the stack having a first input/output pad arranged to provide a signal to a functional circuit of the first integrated circuit chip, and having a second input/output pad uncoupled with respect to providing a signal to functional circuits of the first integrated circuit chip; and an electrical connection coupling the second input/output pad of the first integrated circuit to an input/output pad of a second integrated circuit chip of the stack such that a signal operatively received at the second input/output pad of the first integrated circuit chip from a third input/output pad of the first integrated circuit chip is operatively driven to the input/output pad of the second integrated circuit chip via the electrical connection.
 2. The electrical apparatus of claim 1, where the input/output pad of the second integrated circuit chip is electrically uncoupled from functional circuits of the second integrated circuit chip.
 3. The electrical apparatus of claim 2, where the input/output pad of the second integrated circuit chip is coupled to an input/output pad of a third integrated circuit chip of the stack via an electrical connection.
 4. The electrical apparatus of claim 1, where each integrated circuit chip of the stack is coupled to only directly adjacent integrated circuit chips of the stack by electrical connections.
 5. The electrical apparatus of claim 1, wherein each integrated circuit chip of the stack includes an input/output pad uncoupled with respect to providing a signal to functional circuits of that integrated circuit chip, the uncoupled input/output pads arranged to provide communications among the integrated circuit chips of the stack.
 6. The electrical apparatus of claim 1, wherein the integrated circuit chips are coupled together by a plurality of electrical connections such that only a single input/output pad device driver does not provide all power to transmit a signal to each integrated circuit chip of the stack.
 7. The electrical apparatus of claim 1, wherein the stack includes a plurality of input/output pads uncoupled to functional circuits of the stack, associated control circuitry and device drivers to buffer communications external to the stack.
 8. The electronic apparatus of claim 1, wherein the stack includes a logic chip to direct transfer of signals to memory chips of the stack.
 9. The electronic apparatus of claim 8, wherein the logic chip has a physical area larger than a physical area of each integrated circuit chip in the stack.
 10. An electronic apparatus comprising: integrated circuit chips arranged in a stack; a set of input/output pads on each integrated circuit chip in the stack, the set of input/output pads arranged to provide a signal to a functional circuit of the integrated circuit chip; a set of uncoupled input/output pads on each integrated circuit chip in the stack, the set of uncoupled input/output pads uncoupled from functional circuits of the integrated circuit chip; electrical connections arranged among the integrated circuit chips of the stack such that a total electrical impedance associated with the electrical connections is minimized relative to possible arrangements of the electrical connections.
 11. The electronic apparatus of claim 10, wherein the integrated circuit chips include integrated circuits of different sizes stacked from a bottom to a top of the stack in decreasing size order with an integrated circuit chip having a largest physical area located at the bottom and an integrated circuit chip having a smallest physical area located at the top of the stack.
 12. The electronic apparatus of claim 10, wherein each integrated circuit chip of the stack has an area different from the area of each of the other ones of the integrated circuit chips of the stack.
 13. The electronic apparatus of claim 10, wherein each integrated circuit chip includes signal drivers associated with the input/output pads on the integrated circuit chip, the signal drivers equal in number to the input/output pads for each integrated circuit chip.
 14. The electronic apparatus of claim 10, wherein one of the integrated circuit chips in the stack is structured to direct signals among the integrated circuit chips in the stack.
 15. The electronic apparatus of claim 14, wherein the one integrated circuit chip in the stack structured to direct signals among the integrated circuit chips in the stack is located in a middle location in the stack.
 16. The electronic apparatus of claim 10, wherein the integrated circuit chips in the stack include integrated circuit chips structured as memory devices.
 17. The electronic apparatus of claim 16, wherein the stack includes a microprocessor chip to direct transfer of signals to the memory chips.
 18. A method comprising: arranging integrated circuit chips in a stack; arranging a set of input/output pads on each integrated circuit chip in the stack to provide a signal to a functional circuit of that integrated circuit chip; arranging a set of uncoupled input/output pads on each integrated circuit chip in the stack such that the set of uncoupled input/output pads are uncoupled with respect to providing a signal to functional circuits of that integrated circuit chip; and arranging electrical connections among the integrated circuit chips of the stack such that a total electrical impedance associated with the electrical connections is minimized relative to possible arrangements of the electrical connections.
 19. The method of claim 18, wherein arranging the integrated circuit chips includes arranging the integrated circuit chips according to a decreasing physical area of the integrated circuit chips.
 20. The method of claim 18, wherein arranging the electrical connections includes identifying electrical paths that connect the integrated circuit chips in the stack having larger electrical impedance values than other possible electrical paths and interconnecting the integrated circuit chips by routing portions of the identified electrical paths through some of the uncoupled input/output pads to spread an electrical load and reduce the total electrical impedance.
 21. The method of claim 18, wherein the total electrical impedance associated with the electrical connections is minimized relative to possible arrangements of the electrical connections with respect to arranging a portion of the electrical connections to the set of uncoupled input/output pads. 